Digital computer systems operate on a binary information signal that, at any point in time, is in either a low state or a high state, with the exception of the time during which a transition is made between the low state and the high state. The smallest unit of time within which the binary information signal must be in either a low state or a high state and the basic unit of information is known as a bit. Eight consecutive bits constitute what is known as a byte of information.
Digital computer systems typically employ a record carrier, such as an optical disk, to store binary information for future use. Binary information is stored or written on the record carrier by changing the state of the record carrier from a first state to a second state in accordance with a change in the state of the binary information signal. For example, an optical disk is typically in a reflective state before any information is written thereon. A binary information signal is written or recorded on the optical disk by changing the reflectivity of the optical disk from the reflective state to a non-reflective state in accordance with a change in the state of the binary information signal from its low state to a high state. The change in the state of the optical disk is achieved by increasing the intensity of a laser beam to a point where it removes the reflective surface of the optical disk to form a non-reflective pit. When the binary information signal returns from the high state to the low state, the intensity of the laser beam is reduced to a point where the reflective surface of the optical disk is unaffected. In this fashion, a binary information signal can be recorded on an optical disk.
Record carriers are subject to recording errors where the state of the record carrier is not changed in accordance with a change in the state of the binary information signal. For example, if the laser malfunctions in an optical disk system the necessary pit may not be established on the optical disk when the information signal changes from a low state to a high state.
Record carriers are also subject to media defects that may alter the meaning of the binary information signal recorded on the record carrier. For example, the reflective surface of an optical disk may be pitted at an inappropriate location. If the pit occurs at a location which corresponds with the state of the binary information recorded thereon, then the media defect is of no consequence. If, however, the pit occurs at a location that does not correspond to the state of the binary information signal, then the media defect has created an error.
To determine if there is a recording error or a media defect on a record carrier, the binary information signal is compared to the information established on the record carrier. A known method, employed in magnetic and optical disks, of detecting recording errors and media defects is to write the binary information on the disk during a first revolution of the disk, read the binary information established on the disk during a second revolution of the disk and then compare the "write" binary information to the "read" binary information. If the "write" binary information and the "read" binary information on the disk are not equivalent, then there is either a recording error or media defect. While this method does detect recording errors and/or media defects on the record carrier, the need to write information during a first revolution and read information during a second revolution leads to a low recording rate or throughput. To increase the throughput, known optical disk systems have compared the signal generated by the optical record carrier immediately after a pit has supposedly been established on the optical disk to the corresponding bit of the binary information signal. Exemplary of such an optical disk system is U.S. Pat. No. 4,363,116 which issued to Kleuters, et al., on Dec. 7, 1982, for a "Method, Apparatus, and Record Carrier Body for Optically Writing Information" and is assigned to the majority partner of the assignee of the present invention. In Kleuters, et al., the intensity of the laser beam is increased in accordance with a change in the state of the binary information signal from a low state to a high state. This increase in intensity supposedly establishes a pit in the optical disk. Upon a transition of the write binary information signal from the aforementioned high state to a low state, the intensity of the laser beam is decreased to a point where the reflective surface of the optical disk is unaffected and interaction of the laser beam with the surface of the optical disk produces a signal indicative of the state of the optical disk. Even though the laser beam is now positioned at a point following the pit due to the rotation of the optical disk, the spot size of the laser beam is large enough to interact with the pit and produce a signal known as a pip. The pip is compared to the high state of the binary information signal to verify that the bit or bits of the information signal in the high state have been properly recorded on the optical disk.
Presently, the speed at which information is recorded on optical record carriers, i.e., the data transfer rate, has increased to the point where the aforementioned pip can no longer be used to verify that a pit has been established on the optical disk in accordance with the state of the binary information signal. At least one reason that the pip can no longer be used is the low bandwidth of the preamplifier employed in the read circuitry relative to the bandwidth of the reflected write pulse. High bandwidth preamplifiers are available to remedy this problem. However, such amplifiers are presently difficult to implement and very expensive. The photo-detectors employed in the read circuitry also exhibit a low bandwidth relative to the bandwidth of the reflected write pulse. It is, however, desirable that a high throughput be maintained. Consequently, there is a need for a method and apparatus to verify that a record carrier has been transformed from a first state to a second state in accordance with a binary information signal while also providing a high throughput in a recording system with a high data transfer rate. It is also desirable that the needed method and apparatus employ a relatively low bandwidth preamplifier and photo-detector in the read circuitry.